Make configuration optional

[justinrubek]

A configuration file was added when the only way to update versions was by specifying the files to change. With #3 we now have automatic detection of cargo workspace members. When using this functionality the configuration is minimal and is mostly specifying that we want autodetection. It might be a lot easier to just remove the configuration requirement in this case-- If bomper is ran without a bomp.ron, maybe we can just make use of all automatic methods (assuming more are implemented).